含气泡油滴撞击矩形沟槽壁面的数值分析

采用耦合水平集-体积分数（CLSVOF）方法对含气泡油滴撞击矩形沟槽壁面现象进行数值模拟研究，考察了油滴撞击壁面后的形态演化过程，分析了中心射流形成机理和气体夹带的分布规律，并探究了沟槽宽度、沟槽深度和撞击位置对油滴铺展特性的影响。研究表明：含气泡油滴在矩形沟槽壁面铺展时会形成中心射流，沟槽内部存在气体夹带现象。气泡底部的速度旋涡是形成中心射流的主要原因，沟槽内的气体夹带受油滴铺展速度影响呈现规律性分布。沟槽宽度对含气泡油滴在垂直沟槽方向和平行沟槽方向的铺展长度影响较大，但对铺展高度影响较小。当无量纲沟槽宽度为0.3时，油滴形成颈部射流并在运动后期使垂直沟槽方向的铺展长度迅速增加。此外，沟槽深度也对含气泡油滴在各方向的铺展有重要影响，沟槽深度越大，中心射流现象越难形成。撞击位置变化不改变油滴在沟槽壁面上的运动演化过程，但对沟槽内部的气体夹带规律有一定影响。

Numerical analysis of hollow oil droplet impact on rectangular groove surface

The behavior of the hollow oil droplets impinging on the rough surface often take place in the oil-gas lubricating system, which will affect the quality of the oil film formation on the surface of the friction pairs. The coupled level set and volume-of-fraction (CLSVOF) method is used to simulate this behavior. The formation mechanism of the central jet and the distribution of air entrainment are analyzed. And the effects of grooved width, grooved depth and impact position on the oil droplet spreading process are investigated. The results show that the central jet and air entrainment can be formed after the hollow oil droplet rectangular groove surface. The formation of the central jet originates from the velocity vortex at the bottom of impacting on the bubble. The position of air entrainment in the grooves is influenced by the spreading velocity of the hollow oil droplet. When the dimensionless grooved width A_g is less than 0.2 or equal to 0.4, the spreading coefficient of vertical grooves D_x of the hollow oil droplet is approximately same. However, when the dimensionless grooved width is equal to 0.3, the spreading coefficient of vertical grooves D_x increases rapidly in the later period of movement due to the formation of a neck jet on the grooved surface. In addition, the spreading coefficient of parallel grooves D_z increases with the increase of grooved width. The spreading height coefficient D_y of the oil droplet is consistent on each surface and is not affected by the grooved width. When the dimensionless grooved depth H_g ≤ 0.2, the spreading coefficient of vertical groove D_x decreases with the depth of groove increasing. When the dimensionless grooved depth H_g>0.2, the spreading coefficient of vertical grooves D_x is almost equal. However, with the increase of grooved depth, the spreading coefficient of parallel grooves D_z is decreased. The spreading height coefficient D_y of the hollow oil droplet decreases with the increase of the grooved depth, and the increase of the grooved depth can prevent the formation of the central jet. The effects of the impact position on the distribution of air entrainment are significant, while the movement of hollow oil droplet plays a negligible role in them.